sched: Fix race in task_group()

Stefan reported a crash on a kernel before a3e5d1091c1 ("sched:
Don't call task_group() too many times in set_task_rq()"), he
found the reason to be that the multiple task_group()
invocations in set_task_rq() returned different values.

Looking at all that I found a lack of serialization and plain
wrong comments.

The below tries to fix it using an extra pointer which is
updated under the appropriate scheduler locks. Its not pretty,
but I can't really see another way given how all the cgroup
stuff works.

Reported-and-tested-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1340364965.18025.71.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 8 insertions, 1 deletions

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 536b213f0ce5..5d011ef4c0df 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1096,7 +1096,7 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
          * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.
          *
          * sched_move_task() holds both and thus holding either pins the cgroup,
-         * see set_task_rq().
+         * see task_group().
          *
          * Furthermore, all task_rq users should acquire both locks, see
          * task_rq_lock().
@@ -7658,6 +7658,7 @@ void sched_destroy_group(struct task_group *tg)
  */
 void sched_move_task(struct task_struct *tsk)
 {
+        struct task_group *tg;
         int on_rq, running;
         unsigned long flags;
         struct rq *rq;
@@ -7672,6 +7673,12 @@ void sched_move_task(struct task_struct *tsk)
         if (unlikely(running))
                 tsk->sched_class->put_prev_task(rq, tsk);
 
+        tg = container_of(task_subsys_state_check(tsk, cpu_cgroup_subsys_id,
+                                lockdep_is_held(&tsk->sighand->siglock)),
+                          struct task_group, css);
+        tg = autogroup_task_group(tsk, tg);
+        tsk->sched_task_group = tg;
+
 #ifdef CONFIG_FAIR_GROUP_SCHED
         if (tsk->sched_class->task_move_group)
                 tsk->sched_class->task_move_group(tsk, on_rq);